With the development of mobile communications, multi-band antennas are required to simultaneously support the multi-band and multi-standard wireless systems such as 2G, 3G and 4G. On the other hand, dual polarization is necessary for base station antenna arrays. Therefore, multi-band dual-polarized antenna arrays are demanded in these systems. In multi-band array designs, it is common to use separated antenna elements which operates at different frequency bands. Mutual coupling between the elements becomes a problematic issue, especially when the two frequency bands are close to each other. Although increasing the separation of antenna elements can reduce mutual coupling, the array becomes bulky. Instead, if the out-of-band radiation of the antenna elements can be suppressed, then the mutual coupling can be reduced effectively. This is to say, the antenna elements with filtering behavior are desirable.
Recently, much work on filtering antennas was conducted. In these designs on filtering antenna, it was popular to cascade filtering circuits and antennas. In this way, the last-stage resonator of the filter was replaced by an antenna radiator. Extra impedance transformers were employed between the filter and antenna or optimized impedance was chosen at the interface between them. Satisfying performance has been reported, however the multiple resonators usually occupied large area. The size can be reduced by using defected ground plane and 3-D configurations that putting cavity resonators under the radiators. Besides, compact designs can also be obtained by integrating a filtering power divider or balun filter into antenna feeding networks.
In the above designs, the filtering antenna was realized by integrating extra filtering circuits to the antenna feeding networks. Therefore, insertion loss caused by the extra filtering circuits was inescapable, resulting in lower antenna gain or efficiency. To avoid the problem, a filtering printed unidirectional loop antenna was realized by adding a parasitic loop, and a stacked patch filtering antenna was achieved by using shorting vias and U-slot. Since no particular filtering circuits were involved, the antenna performances were not affected. However, both designs are singly-polarized, and their structures are not easy to be extended for dual polarizations.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.